Billed by chairman Robert Kirby as the best water theme park in the world, Wet 'n' Wild cost $120 million to build, features 40 attractions and will cater for up to 20,000 visitors a day.

Its fibreglass water pipes, laid end-to-end, would stretch 32 kilometres from its home in Prospect to Bondi.

However, Jamberoo Action Park marketing director James Cook said management would not have invested $15 million last year to complete the first stage of the Kangaroo Island project if it was not confident it could increase visitor numbers.

"We certainly indicated our budgeting would require an increase on its visitation from last year and we are very confident we'll achieve that," he said.

Mr Cook acknowledged a high proportion of the park's clientele came from western Sydney - figures from the Kangaroo Island approval process indicated it was as high as 70 per cent - but he welcomed the competition.

"Our view is very much that an entrant like Wet 'n' Wild will actually grow the category that we're in. It's not going to cannibalise the existing pool of people who would be thinking of going to a water theme park but, in actual fact, grow it, because that's what competition does."

The action park's Facebook page recently featured a double-decker bus covered in images of the park driving around western Sydney and Mr Cook said 15 new metropolitan roadside billboard signs had been secured, but it was a strategy the business has used for several years.

"The only difference is there is a water theme park that is being built in an area which we've always been advertising in."

Mr Cook thought Wet 'n' Wild's $8 million advertising campaign would also help Jamberoo Action Park.

"That stimulates a lot of people who might not have been thinking about a water theme park experience to actually give it a try." While season passes generally attract a higher proportion of locals, the action park's offering is cheaper than Wet 'n' Wild's.

The $40 million Kangaroo Island development was originally intended to be a staged four-year process, weather dependent.

The first stage was completed last December, but Mr Cook said it was unclear when the second stage would start.

"Nothing is really set in concrete as to when the masterplan would be concluded. However, if we looked at the last masterplan, it was realised in around a half a dozen years from the time it was commenced."